An in-band on-channel (IBOC) digital broadcasting system simultaneously broadcasts analog and digital signals in a standard AM broadcasting channel. One AM IBOC system is described in U.S. Pat. No. 5,588,022. The broadcast signal includes an amplitude modulated radio frequency signal having a first frequency spectrum. The amplitude modulated radio frequency signal includes a first carrier modulated by an analog program signal. The signal also includes a plurality of digitally modulated subcarriers within a bandwidth that encompasses the first frequency spectrum. Each of the digitally modulated subcarriers is modulated by a digital signal. A first group of the digitally modulated subcarriers lies within the first frequency spectrum and is modulated in quadrature with the first carrier signal. Second and third groups of the digitally modulated subcarriers lie outside of the first frequency spectrum and are modulated both in-phase and in quadrature with the first carrier signal. The subcarriers are divided into primary, secondary and tertiary partitions. Some of the subcarriers are complementary subcarriers.
The digital information is transmitted in successive symbols. Symbols transmitted in the IBOC system must be tracked in the receiver modem to align the demodulation of the symbols with the incoming stream of signal samples. This invention provides a symbol tracking algorithm for use in an HD Radio™ digital modem.